Mineralogy and geochemistry of the black shales from Wufeng-Longmaxi Formations(Late Ordovician-Early Silurian) in Xindi No. 1 well, Southwest of Sichuan Basin: Constraints on provenance, source area weathering and tectonic setting

Abstract The Late Ordovician-Early Silurian period witnessed the second largest Phanerozoic mass extinction, Hirnantian glaciation and volcanic events. Wufeng, Guanyinqiao and Longmaxi formations were developed in South China across the Ordovician-Silurian transition.To infer the weathering conditions, provenance, tectonic setting of the source area in western South China during this period, we measured the mineralogical and geochemical characteristics of 42 samples of Wufeng-Longmaxi formations taken from Xindi No. 1 well, south west of Sichuan Basin. These shales and argillaceous limestones mainly consist of quartz, clay mineral, carbonate, feldspar, and minor amounts of pyrite. Geochemically, the rocks from three formations are enriched in CaO and MgO and slightly depleted in SiO2, Al2O3 and K2O. Chondrite-normalized rare earth elements (REE) patterns display a negative Eu anomaly, a slight enrichment in LREE and a flat HREE. Paleoweathering indices (CIA and PIA) and Th/U ratios indicate that the sediments experienced moderate-intense to weak chemical weathering in the source area. CIA values of the lower Wufeng Formation range from 64.88 to 70.88, indicating a warm and humid climate. Upward the CIA values on the top of Wufeng Formation decrease from 62.56 to 62.93, the CIA values of the Guanyinqiao and lowest Longmaxi formation are consistently low values (58.48-67.12) , suggesting a cold and dry climate which is consistent with Gondwana glaciation in Hirnantian period. Geochemical discrimination diagrams revealed that the rocks of the three formations were derived from felsic igneous source rocks and deposited in an active continental margin setting.